Leucine-rich repeat kinase 2 (LRRK2) is a newly identified gene whose mutations are associated with late onset pathogenesis of Parkinson's disease (PD) and have unprecedented prevalence in both familial and sporadic PD. Thus, LRRK2 provides a unique opportunity for understanding the pathogenesis of PD. Despite this, little is currently known about the function of the LRRK2 or its role in PD pathogenesis. The long-term goal of this program is to unveil the molecular mechanisms by which mutant LRRK2 is involved in the pathogenesis of PD. Our specific approach is to identify interacting proteins that define and regulate the GTPase function of LRRK2. LRRK2 contains multiple catalytic and protein interaction domains which may be involved in integrating signaling pathways crucial for neuronal function. The Roc domain of LRRK2 is reported to have GTPase activity that may intra- molecularly regulates the kinase activity of LRRK2. Using this domain and its associated COR domain as bait, we have recently identified specific interactions of two known signaling proteins with LRRK2, a RacGAP and an antioxidant protein. The interaction with a RacGAP may help to define the function of the Roc domain as well as its regulation. On the other hand, the interaction of LRRK2 with an antioxidant protein may provide a molecular link for a role of LRRK2 in neurodegenerative processes such as oxidative stress. Therefore, we hypothesize that LRRK2 has Rac-like GTPase activity that regulates downstream effectors, such as antioxidant proteins, to play a role in neuronal maintenance and survival. The following specific aims are designed to test this hypothesis: (1) To examine whether LRRK2 has Rac-like GTPase activity that is regulated by a known interacting RacGAP. We will further characterize the specificity of the interaction and investigate if the Rac-like GTPase activity of the Roc domain is regulated by the GAP. We will then determine how the PD pathogenic mutations interfere with this regulation. Finally we will examine if the regulation of LRRK2 GTPase activity regulates its kinase activity. (2) To determine whether LRRK2 regulates an identified interacting antioxidant protein to effect neuronal survival. We will first characterize the specificity of the interaction and examine whether Prx2 is phosphorylated by LRRK2. We will then investigate how the function of the antioxidant protein is regulated by LRRK2 through this interaction. Our proposed studies will not only examine the normal regulation and function of LRRK2 in neuronal signaling but also the role of its PD associated mutants in neuronal pathogenesis. PUBLIC HEALTH RELEVANCE: As a newly identified PD associated gene, leucine-rich repeat kinase 2 (LRRK2) shows striking linkage with both familial and sporadic PD and is associated with typical late-onset pathogenesis. Therefore, our proposed studies on the LRRK2 interacting proteins will not only examine the normal regulation and function of LRRK2 in neuronal signaling but also provide insight into the mechanisms by which the pathogenic mutants are involved in PD pathogenesis. Furthermore, functional understanding of LRRK2 interactions with its signaling partners will likely reveal novel therapeutic targets, in addition to LRRK2 itself, for the treatment of PD.